1. Field of the Invention
This invention relates to a focus detecting device in an optical apparatus such as a photographic camera and particularly to a focus detecting device of a single-lens reflex camera or the like of the kind called the TTL type which is arranged to detect an in-focus state of a photo-taking lens of the apparatus through an imaging light flux coming from the photo-taking lens.
2. Description of the Prior Art
There have been known focus detecting devices of the kind using a so-called image displacement detecting method in which an in-focus state is detected with a defocus degree of an objective computed by detecting displacement in the relative positions of two images of an object formed with light fluxes from two different regions of the exit pupil of an objective. These known devices include, for example, a device disclosed by U.S. Pat. No. 3,875,401, issued Apr. 1, 1975. The focus detecting device of the prior art using the image displacement detecting method is provided with an optical system which is arranged as schematically shown in FIG. 1(a) of the accompanying drawings. Assuming that this device of the prior art is applied to a single-lens reflex camera adapted for use of interchangeable lenses, the device operates as follows: In case of the device shown in FIG. 1(a), the image of an object to be photographed which is formed on a prescribed image plane 2 by imaging light flux portions 5a and 5b coming from peripheral parts of a photo-taking lens 1 is arranged to be once again formed on the surfaces of two light sensitive elements 4a and 4b by two secondary imaging lenses 3a and 3b respectively. Then, the relative positional relation of the two images of the object is detected via output signals of the two light sensitive elements 4a and 4b. The device thus determines whether the photo-taking lens 1 is in an in-focus state or in an out-of-focus state by signals produced from these photo sensitive elements. When the focus point of the photo-taking lens 1 deviates from the prescribed image plane and is thus in an out-of-focus or defocus state, the two images of the object re-imaged on the surfaces of the light sensitive elements 4a and 4b displace from a position obtained when the photo-taking lens is in the in-focus state and are thus located in different positions displaced in the direction perpendicular to an optical axis L. In that instance, the positional relation between the two object images formed on the light sensitive elements 4a and 4b is detected via the light sensitive elements. The defocus degree of the photo-taking lens 1 is thus directly obtained. The lens 1 is then shifted according to the defocus degree to bring it into an in-focus state.
In order that the in-focus state detection is accurately carried out in accordance with the image displacement detecting method, the light sensitive elements 4a and 4b must be arranged to have equal light quantity distribution. The term "equal light quantity distribution" as used here means that, when the object to be photographed has uniform brightness, the light quantity distribution on each of the light sensitive elements 4a and 4b is not only uniform but also equal to that of the other. However, in case where the photo-taking lens 1 mounted on a camera body is interchanged with a different photo-taking lens, the optical system shown in FIG. 1(a) comes to have a different maximum F-number of the photo-taking lens 1 and a different distance between the prescribed image plane 2 and the exit pupil. As a result, it becomes extremely difficult to equalize the light quantity distribution.
Some methods have already been proposed for equalizing the light quantity distribution. For example, the light quantity distribution equalizing methods of the prior art include a method disclosed by U.S. Pat. No. 4,322,616, issued Mar. 30, 1982. In that method, the light quantity distribution is equalized by providing light shielding plates 6a and 6b in front of the secondary imaging lenses 3a and 3b as shown in FIG. 1(b). With the light shielding plates 6a and 6b disposed directly in front of the secondary imaging lenses 3a and 3b, the brightness of the lenses 3a and 3b is substantially darkened to have the light quantity distribution on one of the light sensitive elements 4a and 4b virtually equalized with the other. However, a shortcoming of that method resides in that, in order to efficiently utilize the quantity of light incident on these light sensitive elements 4a and 4b, an aperture 6c formed by the light shielding plates 6a and 6b must be adjusted every time the maximum F-number value of the photo-taking lens 1 and the exit pupil position come to vary.
Another method of the prior art is proposed by Japanese Laid-Open Patent Application No. Sho 55-118019 (laid-open for public inspection Sept. 10, 1980) and is arranged as shown in FIG. 1(c) of the accompanying drawings. As shown, the arrangement includes four secondary imaging lenses 3a, 3b, 3c and 3d and four light sensitive elements 4a, 4b, 4c and 4d which form a total of four pairs of secondary imaging optical systems arranged in the rear of a lens 2a disposed in the vicinity of the image plane 2 of a photo-taking lens 1. The light sensitive elements 4a and 4b located on the inner side relative to the optical axis L and the light sensitive elements 4c and 4d on the outer side relative to the optical axis L are arranged to be selectively used from one to the other according to the increase or decrease of the focal length of the photo-taking lens 1 by means of a switch 7. The outputs of the pairs of the light sensitive elements which equalize the light quantity distribution (according to the above-stated difference of the photo-taking lens 1 mounted on the camera body) are compared by a phase comparison circuit 8 with each other to determine the degree of defocus of the photo-taking lens 1.
This method, however, necessitates use of four secondary imaging lenses 3a-3d and four light sensitive elements 4a-4d which must be arranged in a direction perpendicular to the optical axis L. As a result, the whole optical system becomes larger in size. A focus detection device according to such a method, therefore, has been hardly suited for use within cameras which have severely limited space available therein.
In the focus detecting device according to such an image displacement detecting method, the degree of displacement between the images formed by the secondary imaging optical systems is approximately proportional to the defocus degree of the photo-taking lens. Hence, a speedy automatic focusing operation can be carried out because the amount of movement and the direction of the movement of the photo-taking lens required for focusing can be readily determined from the detected degree of image displacement. Besides, it can rather efficiently detect whether the focused image is positioned forward to the prescribed image plane or backward to the prescribed image plane even in cases where images blur to a great extent on the prescribed image plane. While these are the advantages of the focus detecting device, a shortcoming of the device of this type resides in that: When the object to be photographed has a periodic pattern, a false peak might appear in the correlation processing of the displaced images of this object and the false peak would result in an erroneous action of the focus detecting device.
There is another known detecting method applicable to a focus detecting device. This method is called an image sharpness detecting method. In the image sharpness detecting method, the sharpness of an image formed on the prescribed image plane of the photo-taking lens is detected by a plurality of light sensitive elements arranged in front and in rear of the prescribed image plane in order to detect the focusing condition of the lens. The image sharpness detecting method has a high degree of detection accuracy when the lens is in the vicinity of an in-focus condition. However, by this method it is hardly distinguishable whether the focused image is positioned forward to the prescribed image plane or backward to the prescribed image plane when an image blurs to a great degree on a prescribed image plane with a telephoto lens or the like used.
In view of these advantages and disadvantages of the prior art methods, attempts are now being made to contrive some focus detecting device that is arranged based on both the above-stated detecting methods to have the advantages of the two methods and is free from any of the disadvantages mentioned above for the detection of an in-focus condition. For this purpose, there is a strong demand for some simple optical system that uses light fluxes from a photo-taking lens and is capable of forming images therefrom according to the different detecting methods.